1. Field of the Invention
The present invention relates to a method for determining the flying height of a head of a hard disk drive.
2. Background Information
Hard disk drives contain a plurality of magnetic heads that are coupled to rotating disks. The heads write and read information by magnetizing and sensing the magnetic fields of the disk surfaces. Each head is attached to a flexure arm to create a subassembly commonly referred to as a head gimbal assembly (“HGA”). The HGA's are suspended from an actuator arm. The actuator arm has a voice coil motor that can move the heads across the surfaces of the disks.
HGA transducers include three primary elements: a read sensor, a write element and a head protrusion control element, also known as fly-on-demand (“FOD”). The read sensor is commonly made of an magneto-resistive structure. The write element includes a coil and a magnetic flux path structure made with high permeability and high magnetization material. The head protrusion control element (FOD device) includes a heater coil. When a current is applied, the coil generates heat and causes the write and read elements to move closer to the media.
During operation, each head is separated from a corresponding disk surface by an air bearing. The air bearing eliminates mechanical interference between the head and the disks. The FOD device is used to further set read and write positions above the disk surface, based on a pre-calibrated target. The strength of the magnetic field from the disk is inversely proportional to the height of the read head spacing to the disk. Reduced spacing results in a stronger magnetic field on the disk, and vice versa.
The flying height of a head may vary during the operation of the drive. For example, a shock load on the drive may create a vibration that causes the heads to mechanically resonate. The vibration causes the heads to move toward and then away from the disk surfaces in an oscillating manner. Particles or scratch ridges in the disk may also cause oscillating movement of the heads. The oscillating movement may occur in either a vertical or in-plane direction relative to the flexure arm. Environment changes, such as temperature and altitude can also cause a change in the head flying height.
If oscillation of the heads occurs during a write routine of the drive, the resultant magnetic field from the writer on the disk will vary inversely relative to the flying height of the writer. The varying magnetic field strength may result in poor writing of data. Errors may occur when the signal is read back by the drive.
Knowing and controlling the flying heights of the heads is critical for both disk drive reliability and data integrity. To accurately operate the FOD device and achieve the desirable write and read spacings to the disk, flying height measurement techniques have been developed. The most common technique is to use playback signal components in frequency domain.
There are various methods for controlling the flying height through feedback schemes. For example, the flying height can be controlled by analyzing the 1st and 3rd harmonics from square wave read signals in perpendicular recording, or periodic isolated pulses in longitudinal recording. The third harmonic signals tend be relatively weak and have a poor signal to noise ratio. The low signal to noise ratio can degrade the accuracy of the flying height measurement.